1. Field of the Invention
The present invention relates to a cantilever chip for a scanning probe microscope and, more particularly, to a method of making a cantilever chip of this type having a lever portion and a probe portion formed at the free end of the cantilever portion.
2. Description of the Related Art
Published Unexamined Japanese Patent Application No. 62-130302 (filed by IBM, G. Wienisch) entitled "Method and Apparatus for forming an image of a sample surface" proposes an atomic force microscope (AFM) as a microscope which utilizes techniques including a servo technique of a scanning tunneling microscope (STM) invented by Binning, Rohrer, et al. and which can observe an insulating material on an atomic order, which is difficult with the STM.
The structure of the AFM is similar to that of the STM, and the AFM is evaluated as one of the scanning probe microscopes. In the AFM, a cantilever chip having a sharp projecting portion (probe portion) at its free end is disposed close to a sample to oppose it. The sample is scanned in the X and Y directions while the movement of the cantilever chip which is displaced by a mutual force acting between the atoms in the distal end of the probe portion and the atoms in the sample is electrically or optically measured, and the positional relationship between the sample and the probe portion of the cantilever chip is changed relatively to each other, thereby obtaining surface shape information and the like of the sample in the three-dimensional manner.
Since T. R. Albrecht et al. have proposed an SiO.sub.2 cantilever chip by applying a semiconductor IC manufacturing process (Thomas R. Albrecht, Calvin F. Quate: Atomic resolution imaging of a nonconductor by Atomic force Microscopy J. Appl. Phys. 62 (1987) 2599), the Cantilever chip for a scanning probe microscope has been able to be manufactured with excellent reproducibility and a high precision on the order of .mu.m. An inexpensive cantilever chip can be manufactured in accordance with the batch process. Accordingly, this cantilever chip manufactured by applying the semiconductor IC manufacturing process is most popular.
S. Akamine et al. propose a cantilever chip with a probe portion having a sharp chip distal end of less than 40 nm which is manufactured by applying the semiconductor IC manufacturing process (S. Akamine, R. C. Barrett, and C. F. Quate: Improved atomic force microscope images using microcantitlevers with sharp tips, Appl. Phys. Lett. 57(3), 1990, p. 316). FIG. 12 is a perspective view of a cantilever chip for a scanning probe microscope, and FIG. 13 is a longitudinal sectional view of the same. A cantilever chip 10 is manufactured using a silicon wafer as the starting material, and mainly has a cantilever portion 11 made of silicon nitride, a probe portion 12 made of silicon, and a support portion 13 made of silicon.
FIGS. 14A to 14H show a method of manufacturing this cantilever chip.
A silicon wafer 21 having a surface orientation of (100) with its two surfaces being polished is used (FIG. 14A), and silicon nitride films 22 and 23 are formed on its two surfaces. The silicon nitride film 23 on the lower surface is patterned by photolithography, and anisotropic etching is performed by an aqueous potassium hydroxide solution using the remaining silicon nitride film as the mask. As a result, the thickness of the silicon wafer 21 is decreased to obtain a membrane 24 (FIG. 14B).
Subsequently, a silicon nitride film 25 is deposited on the lower surface of the wafer 21. A resist 26 is formed to coat the upper surface of the wafer 21 and patterned by photolithography (FIG. 14C). Using the resist 26 as the mask, reactive ion etching (RIE) is performed to form a through hole through the film 22, the membrane 24 and the film 25, and then the resist 26 is removed (FIG. 14D). FIG. 14E schematically shows the resultant wafer viewed obliquely from the above.
An oxide film 27 is formed by oxidation on a silicon surface which has been exposed when RIE was performed to form the through hole through the membrane 24 as described above. The silicon nitride film 22 on the surface of the wafer 21 is removed by plasma etching, and anisotropic wet etching is performed by an aqueous potassium hydroxide solution. Etching is stopped by the silicon oxide film 27 and the silicon nitride film 25 (FIG. 14G). Finally, the silicon oxide film 27 is removed by hydrofluoric acid to obtain the cantilever chip 10 for a scanning probe microscope which has a tetrahedral probe portion as shown in FIG. 12 (FIG. 14H).
According to this manufacturing method, when the cantilever portion of a cantilever chip is formed to have a triangular free end, a probe portion is automatically formed on the distal end portion of the free end. In other words, the probe portion can be formed by so-called self alignment. Accordingly, the pattern of the probe portion need not be aligned or exposed separately, and the cantilever chip can be manufactured with high precision.
In the conventional process described above, since etching rate control of wet etching is difficult, it is difficult to set the membrane 24 to have a predetermined thickness by wet-etching the silicon wafer 21 from the lower surface, posing a problem in this process. Hence, an improvement is demanded.
The length of the distal end portion of a cantilever chip is determined by the thickness of the membrane. Accordingly, when a plurality of cantilever chips are to be formed in the same wafer, if the surface distribution of the thickness of the membrane is large, the lengths of the probe portions of the respective cantilever chips differ depending on the thicknesses of the membranes.
The length of the probe portion influences the reproducibility of the surface shape of the sample of an AFM image. If the length of the probe portion is excessively large, a moment applied on the probe portion upon reception of a force perpendicular to the axial direction of the probe portion becomes large, and the AFM image does not correctly reproduce the surface shade of the sample. From these reasons, it is demanded to manufacture a plurality of cantilever chips having probe portions with sharp distal ends and the same probe portion length by the batch process, and to provide a method of manufacturing the same.
In the conventional process described above, a silicon wafer having an orientation of (100) is used as a starting wafer. Thus, the tetrahedral probe portion formed at the free end of the conventional cantilever portion shown in FIG. 12 consists of a (111) plane 15, a surface 16 contacting the cantilever portion, and side surfaces 17 and 18 formed in accordance with the steps shown in FIGS. 14E to 14H. Since the shape of the probe portion is determined by only the thickness of the membrane 24 in FIG. 14B, only the same probe portions can be formed when a plurality of cantilever chips are formed on the same wafer.
A demand has arisen from the users of a scanning probe microscope to selectively use different shapes of the probe portions in order to measure samples having various shapes. For example, the users wish to selectively use probe portions having different aspect ratios. In this case, an aspect ratio is a ratio of the height of, e.g., a probe portion to the typical length of a surface contacting a cantilever portion.
Regarding this demand, conventionally, an angle defined by the two side surfaces (17 and 18 in FIG. 12A) of a probe portion can be arbitrarily determined by the design of the mask for photolithography. However, an angle defined by the (111) plane 15 and the surface 16 contacting the cantilever portion is always 54.74.degree..
Further, since the conventional process described above also includes the initial step of forming a membrane (FIG. 14B), the wafer can cause cracking easily in a later step, resulting in a low yield. Since a great deal of attention need be paid during wafer handling, a time required for the manufacture is prolonged, and thus an increase in cost is inevitable.
In the conventional cantilever chip, since the cantilever portion is constituted by a silicon nitride film, the thickness of the cantilever portion can only be set to a maximum of 1 .mu.m. Therefore, it is difficult to form a cantilever portion having a large spring constant.